1. Field of the Invention
The present invention relates to a power unit mounting device.
2. Description of the Related Art
In a conventional power unit mounting device, a mounting rubber, which has an internal space in which a fluid is enclosed, is fixed between the power unit side base and the chassis side base, and on one base side there is formed a secondary compartment which is divided by a diaphragm and a partition plate. A fluid chamber is formed between this partition plate and the other base side. Also provided in this partition plate is one valve mechanism which moves when subjected to high frequency vibrations and is constrained when subjected to low frequency high amplitude vibrations, and an orifice equipped with a movable plate. This orifice serves to connect the secondary chamber and the fluid chamber.
When high amplitude vibrations in the low frequency range are applied to this conventional type of fluid-containing power unit mounting device, the fluid flowing through the orifice is subjected to a damping force. For low amplitude vibrations at high frequencies, the movable plate on the valve mechanism moves, and the dynamic spring constant of the mounting mechanism is constrained in a specific range. Accordingly, a high degree of damping is provided to prevent engine vibration produced at the resonant frequency, for example, 5 to 13 Hz. When the dynamic spring constant is reduced on a mounting device so that the valve mechanism resonates to prevent that noise at higher frequencies which produce noise (for example, 100 to 170 Hz), at a higher frequency than this, say at a high frequency range of about 250 Hz for example, the resonant point is exceeded and the valve mechanism does not operate well. The dynamic spring constant in the mounting mechanism increases and produces a worsening effect so that the problem arises that high frequency vibration from the combustion of the engine is transmitted to the chassis.
In general, for the valve mechanism a spring system is formed for which, if the equivalent mass is m*, and if the sum of the diaphragm rigidity and mounting rubber rigidity is k, then the resonant frequency (f ) becomes: ##EQU1## If the cross-sectional area of the internal space in the mounting rubber is A.sub.1, the effective cross-sectional area of the valve mechanism A.sub.2, and the fluid moving in the vicinity of the valve mechanism has a mass of m.sub.o at microvibrations, then the equivalent mass m*=m`.sub.o (A.sub.1 /A.sub.2).sup.2.